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PICList Thread
'Power Brown Outs'
1996\04\03@140403 by James Musselman

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Here's a brain buster.
We have a stand alone CO gas monitor (safety equipment) that uses a
PIC16C74.
If the power supply is momentarily shorted (milliseconds), the PIC does
not recover, but locks up in random modes.
We have the Watchdog timer ON and CLRWDT is only given one time in the
main program loop.
Any ideas?  I was massacred today in an engineering meeting as this
instrument was on the verge of its first major shipment.
Thanks.

--
Regards, James Musselman, President

Radix/Cobalt Instruments, Inc.
PO Box 897
Clovis, CA  93613 USA
tel  209-297-9000
fax  209-297-9400

Check out my home page  http://rdx.com

1996\04\03@232838 by Brad Morrow

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On Apr 3, 11:02am, James Musselman wrote:
{Quote hidden}

Sounds like a tough day at the office!

You could try enough bulk capacitance on your board to ride through the short.
Or add circuitry to hold the PIC in reset until the supply voltage recovers.
Motorola makes a part for micro-processor resets.


Regards,

Brad Morrow

--
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Brad Morrow                                      Advanced Systems Division
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1996\04\04@001948 by Todd Peterson

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A possible answer to:
>Here's a brain buster.
>We have a stand alone CO gas monitor (safety equipment) that uses a
>PIC16C74.
>If the power supply is momentarily shorted (milliseconds), the PIC does
>not recover, but locks up in random modes.
>We have the Watchdog timer ON and CLRWDT is only given one time in the
>main program loop.
>Any ideas?  I was massacred today in an engineering meeting as this
>instrument was on the verge of its first major shipment.

James:

What type of oscillator are you using? TTL clock,crystal,etc. Is it still
oscillating after the power glitch? I mention this because if the program
counter (instruction pointer) is being loaded with garbage that is outside
your program loop, the watchdog timer should fire and reset the PIC unless
the clock/oscillator has stopped.

Any more info on what state it is locking up in? How long do you have to
power down to reset the processor?  What value of bypass capacitors are on
the PIC?

Good luck,
Todd Peterson


===========================================================
*** Developers of the PICPlus(TM) Microcontroller Board ***

Todd Peterson, Computer Engineer   (.....tpetersonKILLspamspam@spam@netins.net)
E-LAB Digital Engineering, Inc.

P.O. Box 246
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1996\04\04@002623 by John Payson

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> Here's a brain buster.
> We have a stand alone CO gas monitor (safety equipment) that uses a
> PIC16C74.
> If the power supply is momentarily shorted (milliseconds), the PIC does
> not recover, but locks up in random modes.
> We have the Watchdog timer ON and CLRWDT is only given one time in the
> main program loop.
> Any ideas?  I was massacred today in an engineering meeting as this
> instrument was on the verge of its first major shipment.
> Thanks.

I think the biggest issue to watch for is the possibility that a power
glitch might have 'bizarre' effects on the PIC's registers without trashing
it totally.  For example, if you set the TRIS registers on startup and never
again, they may get glitched without the program knowing it.  If an input
becomes an output, the program could become "blind" to the true state of that
input; if an output becomes an input the program would, without knowing it,
become ineffectual at writing that pin.

This problem can be mitigated for some registers (like TRISA, TRISB, etc.)
by simply reloading the registers periodically.  For other registers, things
may be a little harder.  Your best bet is probably to have the program--just
after the CLRWDT--check the state of the system to ensure that it makes
sense.  If you have some bytes to spare, it may be helpful to have a "second
order" software watchdog.  If the software is supposed to do certain things in
response to certain events, you could create software counters/timers to
monitor such things.  For example, if the system contains a modem which can
only answer the phone when not conducting a measurement (measurements should
normally take 5-15 seconds), a simple software watchdog could periodically
check the state of the phone ring signal.  If the phone has rung five times
within a 45-second period, odds are really good that SOMETHING is going wrong
in the measurement routine.

Note that even with well-programmed software watchdogs, and a hardware watch-
dog to check for extreme conditions, power glitches can still cause system
problems.  If your software can't deal acceptable with registers that get
garbled, you could consider a brownout-reset circuit.  This will be much more
reliable than a watchdog at detecting errors resulting from glitches.  It
won't, however, detect faults due to code glitches so a watchdog is still a
good idea.

1996\04\04@125557 by Andrew Warren

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Todd Peterson <.....PICLISTKILLspamspam.....MITVMA.MIT.EDU> wrote:

> >PIC16C74. If the power supply is momentarily shorted
> >(milliseconds), the PIC does not recover, but locks up in random
> >modes. We have the Watchdog timer ON and CLRWDT is only given one
> >time in the main program loop. Any ideas?
>
> if the program counter (instruction pointer) is being loaded with
> garbage that is outside your program loop, the watchdog timer
> should fire and reset the PIC unless the clock/oscillator has
> stopped.

Todd:

Wrong on both counts:

1.  If the PIC's Vdd line drops below the operational threshold (3V
or so), but DOESN"T fall below 0.7V, the PIC will latch up when
power is restored, and the watchdog timer will NOT reset it.

2.  Under normal circumstances, the watchdog timer works EVEN IF the
clock is stopped.

-Andy

Andrew Warren - EraseMEfastfwdspam_OUTspamTakeThisOuTix.netcom.com
Fast Forward Engineering, Vista, California
http://www.geocities.com/SiliconValley/2499

1996\04\04@142045 by John Payson

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> 1.  If the PIC's Vdd line drops below the operational threshold (3V
> or so), but DOESN"T fall below 0.7V, the PIC will latch up when
> power is restored, and the watchdog timer will NOT reset it.

By this, do you mean by the term "latchup", the "pseudo-SCR" latchup that
hits CMOS devices when their substrate junctions get biased into conduction?
If so, would using an LVI chip prevent it, or is it necessary to ensure that
power dips are always taken down to zero?  I do know I once had a 16C84 chip
latch up on me (don't know why, but the chip was getting warm and drawing
150mA; powering the chip down and back again fixed it).

1996\04\04@152050 by Andrew Warren

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John Payson <PICLISTspamspam_OUTMITVMA.MIT.EDU> wrote:

> > 1.  If the PIC's Vdd line drops below the operational threshold
> > (3V or so), but DOESN"T fall below 0.7V, the PIC will latch up
> > when power is restored, and the watchdog timer will NOT reset it.
>
> By this, do you mean by the term "latchup", the "pseudo-SCR" latchup
> that hits CMOS devices when their substrate junctions get biased
> into conduction? If so, would using an LVI chip prevent it, or is it
> necessary to ensure that power dips are always taken down to zero?

John:

The "CMOS Latchup" problem you're describing DOES occasionally happen
(especially to the microprocessors on the PIC-Master emulator
probes), but it's unrelated to the specific problem I was discussing.

The "Brownout latchup" problem, as far as I know, is simply a symptom
of the PIC's logic circuitry starting to function at a lower voltage
than its RAM and EPROM circuitry.  Those Motorola/Ricoh/Maxim
low-voltage-reset chips solve the problem nicely.

-Andy

Andrew Warren - @spam@fastfwdKILLspamspamix.netcom.com
Fast Forward Engineering, Vista, California
http://www.geocities.com/SiliconValley/2499

1996\04\10@073741 by Kalle Pihlajasaari

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Hi, a late reply that no one else seems to have directly suggested.

> We have a stand alone CO gas monitor (safety equipment) that uses a
> PIC16C74.
> If the power supply is momentarily shorted (milliseconds), the PIC does
> not recover, but locks up in random modes.
> We have the Watchdog timer ON and CLRWDT is only given one time in the
> main program loop.

Try to make the PIC get power before the attachments, so that you do not
have a Vin > VCC as this may cause some of the pins to possibly latch
up (speculation but also a requirement), this seems to be the only
mode that occurs from a power glitch instead of a normal power up from
cold.  Your PIC may be working, just unable to talk.

Cheers
--
Kalle Pihlajasaari     KILLspamkalleKILLspamspamdata.co.za
Interface Products     Box 15775, Doornfontein, 2028, South Africa
+27 (11) 402-7750      Fax: +27 (11) 402-7751

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